Field of the Invention
The present invention relates to gypsum products having exposed surfaces and provides such products with enhanced water resistance.
Description of the Background Art
Gypsum products including gypsum blocks, paperless gypsum board, and also gypsum facings, can suffer from water absorption, especially where the products are intended for use in a wet or damp environment, because such products have exposed surfaces which are susceptible to water ingress.
A standard method of testing “moisture resistant” wallboard requires it to have a water absorption not greater than 5%—for example, expressed as the weight percentage of water absorbed by a 300 mm×300 mm board sample immersed in water at 20° C. for two hours. Such moisture resistant board can be used in situations where there is a risk of limited exposure of the board to moisture.
In order to achieve moisture resistance, it has been suggested that a hydrophobic additive may be incorporated into the plaster slurry employed to form the board.
For example, the hydrophobic additive may be a silicone, as described in our European Patent 957070; the use of such a hydrophobic additive requires thermal curing, and it would be advantageous to avoid the requirement for energy input that such thermal curing implies. Latex copolymers have also been proposed in WO2008/152519, but it is not clear that the resulting plaster has long term water resistance.
It remains desirable to increase the mechanical properties of set gypsum products under wet conditions, and produce such products with lowered open porosity as this property will enhance water-resistance.
In particular, there is a need for water-resistant gypsum products having improved strength properties after prolonged contact with water and which can be produced with a mild drying regime.